Purpose:
Recently, miR-34a was identified as a senescence biomarker of the brain, and our goal was to examine the expression of miR-34a as a possible aging biomarker in the neural tissue of the mouse eye.

Methods:
Mouse eyes (ages 4 mo, 18 mo, and 24 mo) were collected under RNase free conditions, fixed, and processed for paraffin embedding. Eyes were sectioned under RNase free conditions and miRNA expression for miR-34a was evaluated using LNA probes from Exiqon via in situ hybridization (ISH). For the positive control we used U6 probe, and scrambled probe was used as a negative control. Expression in the neuroretina and retinal pigmented epithelium(RPE) were examined.

Results:
All sections were negative for Scrambled probe (both retina and RPE). All sections were positive for U6 (both retina and RPE). MiR-34a expression was found to vary between the 4 mo, 18 mo and 24 mo mouse but was much more intense in the 18 mo and 24 mo compared to the 4 mo (both retina and RPE). In general, expression was see in the ganglion cell layer (GCL), inner nuclear layer (INL) and inner segment (IS) of the rods and cones. In addition, in the 18 and 24 month old mouse, the outer nuclear layer (ONL) and inner plexiform layer (IPL) also demonstrated miR-34a expression.

Conclusions:
There are many regulatory factors involved in the aging of cells, and microRNAs are emerging with an important role. MicroRNAs regulate gene expression at the post-translational level and are involved in many biological processes such as cell cycle control, apoptosis, stem cell differentiation, aging, etc. In the mouse retina and RPE, miR-34a appears to be expressed differentially with age, and this could be an indication of cell senescence. In addition, we conclude that miR-34a could serve as a potential biomarker for retinal aging.